1. The Field of the Invention
The present invention relates to an optical pickup.
2. Description of the Related Art
In a conventional pickup, a quarter wavelength plate is disposed in an optical path to convert linearly polarized light to circularly polarized light to be transmitted toward an optical disk. In this optical configuration, a polarization direction in the backward optical path is different from that in the forward optical path by 90 degrees. Hence if an optical element, such as liquid crystal element, that has a dependency on a polarization characteristics in the incidence direction of light is used for correcting an aberration of light, making the optical element active (that is, the aberration correction is active) requires that the polarization direction in the forward or backward optical path should be consistent with the polarization characteristic of the optical element.
Conventional techniques for achieving the above have been provided as follows.
For example, a first conventional technique is shown in FIGS. 1A and 1B, in which a liquid crystal (LC) panel 1, quarter wavelength plate 2, objective lens 3 are disposed before an optical disk DK. The liquid crystal panel 1 is used as the optical element to correct an aberration caused in the optical path traveling in an optical pickup (refer to Japanese Patent Laid-open (KOKAI) No. 2002-150598). In this configuration of the optical pickup, the polarization direction in the forward optical path is consistent with the orientational direction of the liquid crystal, whereby a phase difference caused in the liquid crystal molecules of the liquid crystal panel 1 act on light flux traveling along the forward optical path.
A second conventional technique is shown in FIGS. 2A and 2B, where, in addition to the liquid crystal panel 1, there is provided a liquid crystal panel 4 whose orientational direction is consistent with the polarization direction in the backward optical path.
A third conventional technique is shown in FIGS. 3A and 3B, where there is provided a liquid crystal panel 1 that is designed to act on both the forward and backward optical paths. In this configuration, if linearly polarized light is obtained on the disk DK, there is no change in the polarized light in each of the forward and backward optical paths. Though disposed is only one liquid crystal panel 1, a phase difference caused by the liquid crystal molecules of the panel 1 will act on light flux in the forward and backward optical paths.
A fourth conventional technique is shown in FIGS. 4A, 4B, 5A and 5B, where there is provided an optical pickup compatible with a DVD and a high-density optical disk. A hologram lens 6 is added to an objective lens 3 used for the high-density optical disk. In the case of using this pickup as a compatible pickup for recording and reproducing bits of information on and from the DVD, a blue laser beam dedicated to the high-density optical disk is differentiated in the polarizing direction by 90 degrees from a red laser beam dedicated to the DVD, so that the polarizing hologram lens 6 will not act when the high-density optical disk is subjected to the recording and reproduction of information thereon or therefrom. In contrast, when bits of information are recorded and reproduced onto and from the DVD, the hologram 6 acts on the light.
However, the above various conventional techniques have still suffered from the various problems as follows.
The first conventional technique has used the quarter wavelength plate 2 to realize the circular polarized light on the surface of the disk DK. The polarized direction in the backward optical path is therefore obliged to be different from that in the forward direction by 90 degrees. The liquid crystal molecules in the liquid crystal plate 1 will not act on the light flux in the backward optical path, thereby leaving the aberration in the detection system. As a result, this influences undesirably the focus servo control and the tracking servo control, providing a deteriorated recording and reproduction characteristic to the optical pickup.
The second conventional technique has used the two liquid crystal plates 1 and 4. Hence a production cost of the optical pickup increases. In addition, it has been required to adopt a higher technical control for minimizing a relative positional shift between the two liquid crystal panels.
In the case of the third conventional technique, no aberration is supplied to the detection system. However, the light on the surface of the disk DK becomes linear polarized light. The performance of recording and reproducing information is therefore apt to be sensitive to the structure of an optical disk, such as depths of pits and depths and shapes of grooves. This leads to the problem that the stability of the recording and reproducing performance tends to deteriorate. Various standards for disks are specified with regard to only disks directed to the circular polarized light. If an optical pickup based on the linear polarized light is used, there arises a problem that an optical disk to be targeted will not operate as expected on the specifications thereof.
Still, the fourth conventional technique requires that the hologram lens 6 act on the light in the forward and backward optical paths in cases where the DVD is subjected to recording and reproducing bits of information. This causes the light on the disk DK to have linear polarization, thus having the same problem as that described about the third conventional technique.
As described above, in cases where the optical pickup employs an optical element (such as liquid crystal (LC) panel and hologram lens) having dependency on the polarization and the optical element is desired to act on the light in both the forward and backward optical paths, the linear polarized light is created on the disk. In consequence, the structures of an optical disk have influence largely upon the recording and reproducing characteristics of the optical pickup, thus deteriorating its playability.
In addition, if it is desired to realize the circular polarized light on the disk, the polarization-dependent element will not act on the light in the forward optical path, because the polarizing direction in the backward optical path differs from that in the forward optical direction by 90 degrees. This requires that both optical elements be arranged, respectively, for the forward and backward optical paths.